When towing a seismic array comprising a plurality of long streamers and source arrays, both the towing vessel and the towed array as a unit and the various units of the tow will be subjected to wind, crosscurrent, waves, etc., affecting the relative position of the various elements of the tow and arrays.
In 4-D geophysical imaging, a 3-D seismic survey may be repeated several times over an area which previously has been surveyed, thereby for example indicating changes in the subsurface formation. In order to succeed, it is important that the various surveys shall be at the same location and that the shots are located as closely as possible to those of the previous surveys. It is also of importance that the various streamers follow the same route as for the previous surveys.
In order to control the position of the various streamers and source arrays, it is well known to provide both the streamers and the units of the source array with adjustable wings, enabling control of the unit in lateral direction with respect to each other, also sometimes used for adjusting the depth.
The winches for super wide ropes, lead-ins and gun umbilical cables allow some adjustment of the inline and cross-line positions, but not independent adjustment of one of the units. The further out the cables are let, the wider the spread will go.
It is also known to adjust the position of the source arrays laterally with respect to the vessel and the streamers by using a winch system.
U.S. Pat. No. 7,415,936 describes such latter seismic survey system which includes a winch having a winch cable connectable to a source array towable between two deflected lead-ins. Further, the system comprises a positioning system for determining a current position of the source array and a control means for adjusting the winch to modify the position of the source array to a desired cross line position. The winches may be attached to the deflected lead-ins or mounted on the tow vessel. The winches exert lateral forces on the source array, derived from the deflected lead-ins, to control the inline position of the source array. By applying an adjusting force to the source array the lateral or cross-line position of the source array may be changed into a desired cross-line position.
There is a need for a solution where the cross-line position of the source array easily and quickly may be changed without being dependent on large power consumption an/or without jeopardizing the operation due to possible wire backlash on the winch drums.